1. Field of the Invention
The present invention relates to circuit devices such as inter-connectors that effectively connect electronic devices and form electronic circuits, and printed wiring boards that densely connect lines and components and a fabrication method thereof that can reduce the number of fabrication steps and contribute to improving the yield of fabrication.
2. Description of the Related Art
When electronic devices and a printed wiring board are connected or printed wiring boards are connected, they may be occasionally connected in the direction of thickness (namely, vertical direction or laminating direction) thereof. As connecting means in the vertical direction or laminating direction (namely, inter-connector technology), an anisotropic conductive adhesive agent is known. The anisotropic conductive adhesive agent is made by dispersing fine conductive particles in an adhesive sheet. When the anisotropic conductive adhesive agent sheet is pressed in the direction of thickness thereof with a predetermined force, the pressed region shows for only vertical direction conductivity. With the characteristics of the anisotropic conductive adhesive agent, for example glass cells of a liquid crystal device and a flexible wiring board are electrically connected. In reality, anisotropic conductive adhesive agent is placed between the glass cells of the liquid crystal device and the flexible wiring board. Then, the glass cells and the flexible wiring substrate are pressed together while heating, and they are adhered together. Conductive particles in the anisotropic conductive adhesive agent are electrically connected to the wiring pattern of the flexible wiring board and the glass cell through one particle or plural particles.
As means for electrically connecting electronic devices, lines are electrically connected to electronic devices disposed on the surface of a printed wiring board by using an anisotropic conductive adhesive agent. In this case, the electronic devices can be two-dimensionally connected over the printed wiring board.
In a printed wiring board, namely a double-sided printed wiring board or multi-layer printed wiring board, wiring layers such as double-sided conductive patterns are electrically connected in the following method. In the case of a double-sided printed wiring board, a double-sided copper clad substrate is drilled at predetermined positions. All the surfaces of the substrate including the inner hole walls are chemically plated and then electrically plated so as to form a conductive layer. The conductive layer is electrically plated so as to thicken the conductive layer. Thus, the reliability of the electrical connections of the wiring layers is completed.
In the case of a multi-layer printed wiring board, copper foils layered over both the surfaces of the board are patterned so as to fabricate a double-sided wiring board. Other copper foils are laminated on the double-sided wiring board through respective insulating sheets (or prepregs). By heating and pressuring, the copper foils are incorporated into the board. Then, like the double-sided printing wiring board mentioned above, the laminated board is drilled and plated so as to electrically connect the wiring layers to each other. Further the outer copper foils are patterned. Thus, the fabrication of the four-layer printed wiring board is completed. A multi-layer printed wiring board with more wiring layers can be fabricated by increasing the number of double-sided printed wiring boards.
As a fabrication method of the printed wiring board, a method that electrically connects wiring layers each other without a plating process is known. In detail, a double-sided copper clad substrate is punched or drilled so as to form holes at predetermined positions. The punched or drilled holes are poured with conductive paste by for example printing method. A resin component of the conductive paste is hardened so as to electrically connect the wiring layers each other.
However, the circuit devices (such as above-mentioned inter-connectors, and printed wiring boards) and the fabrication method thereof have the following problems. When an anisotropic conductive adhesive agent is used in the inter-connector technology for connections in vertical direction or laminating direction, connecting resistance is generally high. Thus, the inter-connector technology is not suitable for connections in electric circuits that require low electric resistance. In other words, electric connections that use the above-mentioned anisotropic conductive adhesive agent are restricted.
On the other hand, in the case of the printed wiring board with wiring layers that are electrically connected by plating method and the fabrication method, walls of drilled holes that electrically connect wiring layers should be plated. Thus, the fabrication process of the printed wiring boards redundantly becomes long and the process management becomes complicated. In addition, when conducive paste is poured in holes that electrically connect wiring layers to each other by a printing method, the punching or drilling process is required as with the plating method. Moreover, it is difficult to equally pour the conductive paste in the punched or drilled holes. In addition, the reliability of the resultant electrical connections is low. Thus, the drilling process, plating process, and the like adversely affect the cost, yield, and so forth of the final printed wiring boards.
In the case of the printed wiring board with wiring layers that are electrically connected each other, holes for conductors which connect wiring layers each other are formed on both front and rear surfaces of the printed wiring board. Thus, circuit lines and electronic devices cannot be formed and disposed at the hole positions. In other words, the holes adversely affect the improvement of mounting density of electronic devices and lines. That is, the printed wiring boards fabricated by the conventional method are not suitable for high density of lines and circuit devices. Thus, practical printed wiring boards and a fabrication method thereof that satisfy high density of lines and circuit devices with low cost are needed.